Athermal optical device

ABSTRACT

An athermal optical device and a method for producing the device, such as an athermal optical fiber reflective grating ( 20 ), are described. The athermal optical fiber reflective grating device ( 20 ) comprises a negative expansion substrate ( 22 ), an optical fiber ( 24 ) mounted on the substrate ( 22 ) surface, and a grating ( 26 ) defined in the optical fiber ( 24 ). The method for producing the athermal optical fiber reflective grating ( 20 ) device comprises providing a negative expansion substrate ( 22 ), mounting an optical fiber ( 24 ) with at least one reflective grating ( 26 ) defined therein onto the substrate ( 20 ) upper surface, and affixing the optical fiber ( 24 ) to the substrate ( 22 ) at at least two spaced-apart locations ( 30, 32 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. national phase of International Application No.PCT/US96/13062, filed Aug. 7, 1996. This application claims the benefitunder 35 USC §119(e) of U.S. Provisional Application No. 60/010,058,filed Jan. 16, 1996.

FIELD OF THE INVENTION

This invention relates to a temperature compensated, athermal opticaldevice and, in particular, to an optical fiber reflective grating deviceincorporating a negative expansion beta-eucryptite substrate to producean athermal optical fiber reflective grating device, and to a method ofproducing the athermal optical fiber reflective grating device.

BACKGROUND OF THE INVENTION

Index of refraction changes induced by UV light are useful in producingcomplex, narrow-band optical components such as filters and channeladd/drop devices. These devices can be an important part ofmultiple-wavelength telecommunication systems. The prototypicalphotosensitive device is a reflective grating (or Bragg grating), whichreflects light over a narrow wavelength band. Typically, these deviceshave channel spacings measured in nanometers.

There are already known various constructions of optical filters, amongthem such which utilize the Bragg effect for wavelength selectivefiltering. U.S. Pat. 4,725,110 discloses one method for constructing afilter which involves imprinting at least one periodic grating in thecore of the optical fiber by exposing the core through the cladding tothe interference pattern of two ultraviolet beams that are directedagainst the optical fiber at two angles relative to the fiber axis thatcomplement each other to 180°. This results in a reflective gratingwhich is oriented normal to the fiber axis. The frequency of the lightreflected by such an optical fiber with the incorporated grating filteris related to the spacing of the grating which varies either with thestrain to which the grating region is subjected, or with the temperatureof the grating region, in a clearly defined relationship, which issubstantially linear to either one of these parameters.

For a uniform grating with spacing L, in a fiber with an effective indexof refraction n and expansion a, the variation of center reflectivewavelength, l_(r) is given by

di _(r) dT=2L[dn/dT+na]

In silica and germania-silica fiber reflective gratings the variation incenter wavelength is dominated by the first term in the brackets, thechange of index of refraction with temperature. The expansion termcontributes less than ten percent of the total variability. dl_(r)/dT istypically 0.01 nm/° C. for a grating with a peak reflectance at 1550 nm.

One practical difficulty in the use of these gratings is their variationwith temperature. In as much as the frequency of the light reflected bythe fiber grating as varies with the temperature of the grating regionthis basic filter cannot be used in applications where the reflectedlight frequency is to be independent of temperature. Methods ofathermalizing the fiber reflective grating would increase theapplications for such gratings.

One method of athermalizing a fiber reflective grating is to thermallycontrol the environment of the grating with an actively controlledthermal stabilization system. Such thermal stabilization is costly toimplement and power, and its complexity leads to reliability concerns.

A second athermalization approach is to create a negative expansionwhich compensates the dn/dT. Devices which employ materials withdissimilar positive thermal expansions to achieve the required negativeexpansion are known.

U.S. Pat. No. 5,042,898 discloses a temperature compensated, embeddedgrating, optical waveguide light filtering device having an opticalfiber grating. Each end of the fiber is attached to a different one oftwo compensating members made of materials with such coefficients ofthermal expansion relative to one another and to that of the fibermaterial as to apply to the fiber longitudinal strains, the magnitude ofwhich varies with temperature in such a manner that the changes in thelongitudinal strains substantially compensate for these attributable tothe changes in the temperature of the grating.

Yoffe, G. W. et al in “Temperature-Compensated Optical-Fiber BraggGratings” OFC'95 Technical Digest, paper W14, discloses a device with amechanical arrangement of metals with dissimilar thermal expansionswhich causes the distance between the mounting points of an opticalfiber to decrease as the temperature rises and reduce the strain in agrating.

Such devices have several undesirable properties. First, fabricating areliable union with the fiber is difficult in such devices. Second, themechanical assembly and adjustment of such devices make them costly tofabricate. These systems also show hysteresis, which makes theperformance degrade under repeated thermal cycling. Finally some of theapproaches require that the grating, which can be several centimeterslong, be suspended, making them incompatible with other requirements ofpassive devices such as insensitivity to mechanical shock and vibration.

Another method of incorporating negative expansion which may beenvisaged is to provide a substrate for mounting the optical fibergrating thereon which is fabricated from material with an intrinsicnegative coefficient of expansion.

U.S. Pat. No. 4,209,229 discloses lithium-alumina-silica type ceramicglasses, particularly those having stoichiometries, on a mole ratiobasis, in the range of 1 Li₂O: 0.5-1.5 Al₂O₃: 3.0-4.5 SiO₂, which areparticularly adapted for use as protective outer layers over fusedsilicas and other cladding materials for optical fiber waveguidemembers. When these lithium aluminosilicate glasses are cerammed, thatis, heat treated to produce nucleated crystallizations, the dominantcrystal phase developed is either beta-eucryptite or beta-quartz solidsolution. Nucleating agents such as TiO₂ and ZrO₂ are used to initiatecrystallization of the glass. The glasses produced in this manner havenegative coefficients of expansion averaging about −1.4×10⁻⁷/° C. overthe range of 0°-600° C. Thin layers of these lithium aluminosilicateglasses can be cerammed to develop fine-grained crystal phases by heattreating a coated filament at 700-1400° C. for a time not exceeding oneminute. The cooled outer layer exerts a compressive stress on the coatedfiber.

U.S. Pat. No. 5,426,714 disclose optical fiber couplers which utilizebeta-eucryptite lithium aluminosilicates having a low or negativecoefficient of thermal expansion as fillers for polymeric resins. Theglass-ceramics were obtained by melting the composition in a platinumcrucible at 1650° C. The glass was then drigaged, cerammed and ground toa powder. A beta-eucryptite composition of 15.56 wt.% Li₂O, 53.125 wt.%Al₂O₃, 31.305 wt.% SiO₂ having a negative coefficient of thermalexpansion of −86×10⁻⁷/° C. measured between −40° C. and +80° C. isdisclosed (Col. 4, fines 24-28).

It is an object of this invention to provide a temperature compensatedoptical device which is an athermal device.

It is an object of this invention to provide a temperature compensatedoptical fiber reflective grating device which is an athermal device.

It is an object of this invention to provide a temperature compensatedoptical fiber reflective grating device which tolerates shock andvibration.

It is an object of this invention to provide a temperature compensatedoptical fiber reflective grating device which has a stable centerwavelength.

It is an object of this invention to provide a temperature compensatedoptical fiber reflective grating device in which the grating region ofthe fiber is straight.

SUMMARY OF THE INVENTION

Briefly stated the invention provides a method for producing an athermaloptical device comprising; providing a negative expansion substratehaving an upper surface; mounting a thermally sensitive, positiveexpansion optical component onto the substrate upper surface andaffixing the component to the substrate at at least two spaced apartlocations.

In another aspect of the invention there is provided an athermal opticaldevice comprising; a negative expansion substrate having an uppersurface; a thermally sensitive, positive expansion optical componentaffixed to the substrate upper surface at at least two spaced apartlocations.

In another aspect of the invention there is provided a method forproducing an athermal optical fiber grating device comprising; providinga negative expansion substrate having an upper surface and first andsecond ends; mounting an optical fiber with at least one grating definedtherein onto the substrate upper surface such that the grating liesbetween and at a distance from each end; and affixing the optical fiberto the substrate at at least two spaced apart locations.

In another aspect of the invention there is provided an athermal opticalfiber grating device comprising; a negative expansion substrate havingan upper surface and first and second ends; an optical fiber affixed tothe substrate upper surface at at least two spaced apart locations; anda grating defined in the optical fiber between and at a distance fromeach end.

The novel aspects of this invention are set forth with particularity inthe appended claims. The invention itself, together with further objectsand advantages thereof may be more fully comprehended by reference tothe following detailed description of a presently preferred embodimentof the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an embodiment of an athermal opticalfiber grating device.

FIG. 2 is a schematic drawing of a second embodiment of an athermaloptical fiber grating device.

FIG. 3 is a schematic drawing of a third embodiment of an athermaloptical fiber grating device.

FIG. 4 is an enlarged view of the affixing channel depicted in FIG. 3.

FIG. 5 is a schematic drawing of a fourth embodiment of an athermaloptical fiber grating device.

FIG. 6 is a thermal expansion graph of a beta-eucryptite glass-ceramic.

FIG. 7 is a thermal expansion graph of a beta-eucryptite glass-ceramic.

FIG. 8 is a graph of an athermalized grating center wavelength.

FIG. 9 is a schematic drawing of an embodiment of an athermal opticalfiber fiber coupler device.

FIG. 10 is a schematic drawing of an embodiment of an athermal planarwaveguide device.

DETAILED DESCRIPTION OF THE INVENTION

Thermally sensitive optical devices of the invention include opticalwaveguides, UV photo induced fiber gratings and optical fiber couplers.The optical fiber reflective gratings used in the device of thisinvention are well known to those familiar with the art, for example, UVphoto induced gratings of the Bragg type.

In this invention, the athermalization approach taken is to create anegative expansion which compensates for the positive change inrefractive index of the optical fiber with a change in temperature. Thecoefficient of expansion required is on the order of −50×10⁻⁷/° C., orperhaps slightly higher because of stress-optic effects. In thisapproach, the fiber containing the grating is mounted, preferably undertension, on a substrate that imparts a negative thermal expansion to thefiber. Thus, as the temperature is increased, the tension is reduced,but the fiber is never put into compression (as this would bemechanically unstable).

The optical fiber, for example a germania-silica fiber, is affixed to asubstrate with an intrinsic negative coefficient of expansion. Theincrease of the index of refraction of the fiber caused mostly by thethermal drift, is compensated by a negative mechanical expansion. Thenegative expansion is imparted by a substrate fabricated from a materialbased on a silica based glass-ceramic possessing an intrinsic negativecoefficient of expansion. The negative expansion is obtained by inducingmicro crystals in the glass-ceramic which undergo a reconstructive phasechange on heating at high temperatures,for example about 1300° C., toproduce a highly ordered beta-eucryptite (i.e. stuffed beta-quartz)structure.

A suitable material for the substrate, beta-eucryptite, has beenidentified which provides compensation over a wide temperature range,for example −40° to +85° C., which is mechanically robust against creepand shows minimal thermal hysteresis. In some applications an even widerrange of temperatures may be tolerated. The beta-eucryptite material isbased on a highly ordered lithium aluminosilicate glass-ceramic whichis, in itself, a stuffed derivative of beta-quartz containing aluminumand lithium. Significant titania, for example >2 wt%, is also requiredto be present as a nucleating agent to induce crystallization of thesolid solution in order to minimize grain size and reduce hysteresis dueto inter granular micro cracking.

The beta-eucryptite solid solution of preference lies betweenstoichiometric LiAlSiO₄ (Li₂O:Al₂O₃:2SiO₂=1:1:2) and Li₂Al₂Si₃O₁₀(Li₂O:Al₂O₃:3SiO₂=1:1:3), and the nucleating agents TiO₂ and,optionally, ZrO₂, are added in such a way as to produce accessory phasesAl₂TiO₅ or ZrTiO₄, preferably the former, for the lowest thermalexpansion coefficients.

This glass-ceramic has a true negative expansion micro crystallinephase, strongly along one axis, c-axis, mildly positive along the other,a-axis and is mechanically stable over a wide temperature range, showinglittle hysteresis or physical property degradation.

In weight percent, a suitable glass-ceramic composition range is asfollows: SiO₂43-55%, Al₂O₃31-42%, Li₂O 8-11%, TiO₂2-6%, and ZrO₄0-4%.

The beta-eucryptite substrate of the invention is preferably a materialwith a coefficient of thermal expansion between −30×10⁻⁷/° C. and−90×10⁻⁷/° C., more preferably −50×10⁻⁷/° C. to −75×10⁻⁷/° C., even morepreferably −55×10⁻⁷/° C.

In order to produce material with this degree of negative expansion thebeta-eucryptite has to be very highly ordered to form alternating AlO₄and SiO₄ tetrahedra. This is achieved by heating the crystallized phaseat a top temperature near 1300° C. for at least 3 hours, preferablyabout 4 hours. In order to prevent cracking of the glass a thermalschedule is used which requires heating the glass through a range oftemperatures which maintains a desired viscosity during crystallizationnear 5×10¹⁰ poises thereby precluding sagging or cracking.

The beta-eucryptite materials of the prior art were not obtained in aslab form but rather were prepared as thin coatings or crushed powders.In order to produce a glass-ceramic substrate of the desired size(potentially several centimeters long) a glass of some stability isrequired. The molten glass must be cast into thin slabs, for example<0.5 in. thick, onto a metal table or mold to ensure rapid cooling. Theglass is then annealed at about 700-800° C. for several hours and thencooled slowly to avoid undesirable stresses.

EXAMPLES OF BETA-EUCRYPTITE COMPOSITIONS Example 1

A composition containing on a weight percent basis 50.3% SiO₂, 36.7%Al₂O₃, 9.7% Li₂O and 3.3% TiO₂ is melted at 1600° C. in a crucible thenthe glass is cast onto a cold steel plate to form a disc of about 0.25to 0.5 in thick. The slab is then cut into bars and heated to 715° C. at300° C./hr, to 765° C. at 140° C./hr, to 1300° C. at 300° C./hr, held atthis temperature for 4 hours then cooled at the furnace cooling rate forseveral hours to less than about 100° C.

FIG. 6 shows a thermal expansion measurement on a 2 inch (50 mm) sampleof the material composition of Example 1 which gives an average negativecoefficient of expansion of −78×10⁻⁷/° C. (measured between 25°−150° C.)and a moderate level of hysteresis as evidenced by the very similarheating and cooling curves.

Example 2

A composition containing on a weight percent basis 49.0% SiO₂, 37.1%Al₂O₃, 9.6% Li₂O and 4.3% TiO₂ is melted at 1600° C. in a crucible thenthe glass is cast onto a cold steel plate to form a disc of about 0.25to 0.5 in (6.3 mm to 12.7 mm) thick. The slab is then cut into bars andheated to 715° C. at 300° C./hr, to 765° C. at 140° C./hr, to 1300° C.at 300° C./hr and held at this temperature for 4 hours, then cooled atthe furnace cooling rate for several hours to less than about 100° C.The cooled bar is subjected to four cycles of reheating to 800° C. andcooling to ambient temperatures to minimize hysteresis.

Example 3

A composition identical to that of Example 2 was treated to the sameconditions except that it is held at 1300° C. for only 0.5 hours beforecooling, and it was not subjected to further heating cycles.

FIG. 7 shows a thermal expansion measurement on the material compositionof Examples 2 and 3. Example 2 shows an average negative coefficient ofexpansion of −52.8×10⁻⁷/° C. (measured between 25°-150° C.) andessentially no hysteresis as evidenced by the very similar heating andcooling curves. Example 3 shows zero expansion over the same temperaturerange without hysteresis.

In order to obtain the desired degree of negative expansion it ispreferable that the composition be maintained at the top temperature of1300° C. for about 3 to 4 hours to obtain a highly ordered crystalphase. It is evident that the material of Example 3 which was onlymaintained at 1300° C. for 0.5 hour has a zero coefficient of expansionand is still relatively disordered.

The heat recycling steps are not essential for achieving satisfactoryhysteresis. However, 1 to 4 heat recycling steps may be beneficial. Theheating rate is about 300° C. per hour and the bar is maintained at 800°C. for about 1 hour each cycle. Referring to FIG. 1 there is illustrateda first embodiment of the invention.

The optical fiber reflective grating device 20 has a substrate 22 formedfrom a flat block of a negative expansion material, such asbeta-eucryptite. An optical fiber 24 having at least one UV-inducedreflective grating 26 written therein is mounted on the surface 28 andattached at either end of the surface at points 30 and 32. It isimportant that the fiber is always straight and not subject tocompression as a result of the negative expansion and thus the fiber isusually mounted under tension. Before attachment the fiber is placedunder a controlled tension, as shown schematically by the use of aweight 34. The proper choice of tension assures that the fiber is notunder compression at all anticipated use temperatures. However, thefiber can be under tension at all anticipated use temperatures. Therequired degree of tension to compensate for the negative expansion in aparticular application can readily be calculated by those with skill inthis art.

The attachment material could be an organic polymer, for example anepoxy cement, an inorganic frit, for example ground glass, ceramic orglass-ceramic material, or a metal. In one embodiment the fiber istacked to the substrate with a UV-cured epoxy adhesive. Mechanical meansfor attaching the fiber can also be used.

Generally the optical fiber reflective grating is supplied with acoating material surrounding the fiber. In the preferred packagingapproach the coating in the grating region of the fiber is left intactwhile it is removed in the substrate attachment region at each end ofthe grating. However, the device can have the coating completely removedbetween the attachment locations. Removal of the coating can beaccomplished by one of two methods : a non-contact, non-chemicalstripping mechanism or by conventional chemical stripping.

In another embodiment, FIG. 2, the fiber is not attached directly to thesubstrate. Bonding pads 40, 42 made from a material differing from thesubstrate, for example a glass or a ceramic, are attached to thesubstrate at either end. The fiber 26 is mounted to the pads at points44, 46. These pads afford better attachment properties of the pad to thefiber than could be achieved from the substrate directly to the fiberbecause of the large thermal expansion mismatch. Suitable pad materialshave a coefficient of thermal expansion intermediate between that of thefiber and the substrate for example between −50 and +5×10⁻⁷, preferablyabout −20×10⁻⁷. Alternatively the pad could be a fused silica with acoefficient of expansion closely matching that of the fiber. The padallows the stress of this joint induced by both the thermal mismatch andthe tension of the fiber, to be spread out over a wider area, lesseningthe chances of cracking and detachment. The attachment materials for thefiber and pad connections are similar to those used for mounting thefiber directly to the substrate, for example, an epoxy cement, aninorganic frit, for example ground glass, ceramic or glass-ceramicmaterial, or a metal.

It In another embodiment, FIG. 3, the negative expansion of thesubstrate material 22 is used to create a clamping force on the fiber.The attachment feature, which might be a hole or channel 50, 52 in araised portion 54, 56 of the substrate, is formed in the substrate atroom temperature with a gap that is very slightly smaller than thefiber. Referring to FIG. 4, by lowering the temperature to a point lowerthan any anticipated use temperature, the substrate expands and allowsthe insertion of the fiber 24 into the channel 50. Warming of thesubstrate then causes substrate contraction and creates a clamping forcefor holding the fiber in the channel.

In another embodiment, FIG. 5, the fiber 24 is attached to the substrateat points 30, 32 and the intermediate fiber length 60 is cushioned by alow modulus damping material 62. This low-modulus material, for examplea silicone rubber coating surrounding the fiber or a pad of a siliconerubber, a natural or synthetic rubber or mixtures thereof, between thefiber and the substrate protects the fiber reflective grating againstexternal perturbations such as mechanical shock or vibration. Bowing ofthe fiber is also minimized. In one embodiment the low modulus materialis adhesively attached to the fiber and the substrate.

Mounting the fiber under tension will alter the optical properties ofthe device (for example, the center wavelength of a grating). This canbe addressed by biasing the device with a reflective grating writtentherein to account for the tension, or it can be done by mounting afiber, for example a germania doped silica fiber, without a reflectivegrating written therein under tension and then exposing the fiber to UVlight in order to fabricate the grating in the device in situ.

In a typical embodiment of the invention the temperature sensitivity ofthe center wavelength is about 0.0125 nm/° C., the stress sensitivity ofthe center wavelength is 0.125 nm shift for 9 g of tension, the barefiber has a diameter of 125 microns, a coated fiber has a diameter of250 microns. The strength of the fiber is >200 kpsi and therefore has avery high reliability.

Example Of An Athermalized Grating On A Beta-Eucryptite Substrate

The grating was written in a photorefractive-sensitive fiber, CorningSME-228 fiber, and the fiber was hydrogen loaded at 100 atmospheres in ahydrogen chamber for one week. After removal of the fiber from thehydrogen chamber, a length of approximately 30 mm of coating was removedby mechanical stripping and the fiber was exposed to 240 nm laserirradiation to create the grating. The fiber was then as mounted to asubstrate of beta-eucryptite, prepared essentially according to themethod of Example 2, under a 10 kpsi tension using a UV-curable epoxyadhesive. The assembled grating was heated to 125° C. for 2 hours to outdiffuse any remaining hydrogen and to eliminate low stability UV-inducedtraps. The fiber was thermally cycled between −40° C. and +125° C. Areference fiber was treated in exactly the same way, except it was notattached to a substrate. The grating center wavelength (FIG. 8) variesby approximately 1.9 nm from −40° C. to +125° C. when not attached tothe substrate and by only 0.2 nm when attached to the substrate.

Although this invention has been described for UV photo induced gratingsit can also be applied to the packaging of other thermally sensitivedevices. For instance, optical fiber couplers and optical waveguidescould be athermalized by attachment to a negative expansion substrate.

An optical fiber fused coupler has two or more fibers fused together atone or more points along their length and is mounted on a substrate.Such couplers are thermally sensitive which results in a certain amountof thermal instability. Especially sensitive are biconically taperedcouplers in which interferometric effects. are used, for example aMach-Zehnder interferometer. Such couplers can be athermalized bymounting the coupler to a negative expansion substrate, such as thebeta-eucryptite described in Example 2 above. Referring to FIG. 9 thereis illustrated a fused biconical coupler device 70 which includes anegative expansion substrate 72 to which are mounted two fibers 74, 76.The fibers are fused together at regions 78, 80. The fibers are attachedto the substrate near the ends at locations 82, 84 in the same manner asdescribed above for the optical fiber reflective grating.

Waveguides can be defined, for example, in optical fibers or planarsubstrates. Such waveguides are thermally sensitive which results in acertain amount of thermal instability. Such waveguides can beathermalized by mounting the waveguide to a negative expansionsubstrate, such as the beta-eucryptite described in Example 2 above.Referring to FIG. 10, there is illustrated a planar waveguide device 90which includes a negative expansion substrate 92 on which is adhesivelymounted a layer of material 94 in which a planar waveguide 96 isfabricated by methods well known to those skilled in this art. Thewaveguide material can be, for example, a doped silica such as agermania silicate, other suitable glass compositions, polymers andsemiconductors, including semiconductors with gain, such as laserdiodes.

The device of this invention is a completely passive system andmechanically simple, and demonstrates athermalization. The method ofproducing the device is advantageous because it provides temperaturecompensated optical devices which tolerate shock and vibration and arethermally stable.

While the invention has been described in connection with a presentlypreferred embodiment thereof, those skilled in the art will recognizethat many modifications and changes may be made therein withoutdeparting from the true spirit and scope of the invention, whichaccordingly is intended to be defined solely by the appended claims.

What is claimed is:
 1. A method for producing an athermal optical devicecomprising: (a) providing a negative expansion substrate having an uppersurface; (b) providing a thermally sensitive, positive expansion opticalcomponent including a grating; (c) mounting said thermally sensitive,positive expansion optical component onto the substrate upper surface;and (d) affixing the component to the substrate at at least two spacedapart locations; wherein: (i) in step (a), the negative expansionsubstrate is selected to provide thermal compensation to the thermallysensitive, positive expansion optical component; and (ii) the substratecomprises a beta-eucryptite glass-ceramic.
 2. The method according toclaim 1, in which the beta-eucryptite comprises SiO₂ 43-55%, Al₂O₃31-42%, Li₂O 8-11%, TiO₂ 2-6%, and ZrO₄0-4% on a weight percent basis.3. The method according to claim 1, wherein the component is affixed bya layer of attachment material.
 4. The method according to claim 3,wherein the attachment material is one of a polymer, a frit and a metal.5. A method for producing an athermal optical device comprising. (a)providing a negative expansion substrate having an upper surface; (b)providing a thermally sensitive, positive expansion optical componentincluding a coupler, said coupler comprising at least two optical fibersfused together at one or more points along their lengths; (c) mountingsaid thermally sensitive, positive expansion optical component onto thesubstrate upper surface; and (d) affixing the component to the substrateat at least two spaced apart locations; wherein: in step (a), thenegative expansion substrate is selected to provide thermal compensationto the thermally sensitive, positive expansion optical component.
 6. Themethod according to claim 5, wherein the substrate comprises abeta-eucryptite glass-ceramic.
 7. The method according to claim 6, inwhich the beta-eucryptite comprises SiO₂ 43-55%, Al₂O₃ 31-42%, Li₂8-11%, TiO₂ 2-6%, and ZrO₄ 0-4% on a weight percent basis.
 8. The methodaccording to claim 5, wherein the component is affixed by a layer ofattachment material.
 9. The method according to claim 8, wherein theattachment material is one of a polymer, a frit and a metal.
 10. Amethod for producing an athermal optical fiber grating devicecomprising: (a) providing a negative expansion substrate having an uppersurface and first and second ends; (b) mounting an optical fiber with atleast one grating defined therein onto the substrate upper surface suchthat the grating lies between and at a distance from each end; and (c)affixing the optical fiber to the substrate at at least two spaced apartlocations; wherein: (i) in step (a), the negative expansion substrate isselected to provide thermal compensation to the grating; and (ii) thesubstrate comprises a beta-eucryptite glass-ceramic.
 11. The methodaccording to claim 10, further comprising applying a sufficient tensionto the optical fiber before the affixing step to maintain the fiberunder tension at all anticipated use temperatures.
 12. The methodaccording to claim 10, wherein the optical fiber is affixed to thesubstrate upper surface at a location between the grating and the firstend and at a location between the grating and the second end.
 13. Themethod according to claim 12, wherein the fiber is affixed by a layer ofattachment material.
 14. The method according to claim 13, wherein theattachment material is one of a polymer, a frit and a metal.
 15. Themethod according to claim 14, wherein the polymer is an epoxy adhesive.16. The method according to claim 12, wherein the affixing stepcomprises: bonding a pad of material having a coefficient of expansionintermediate between that of the fiber and the substrate to the uppersubstrate surface at each affixing location and affixing the fiber toeach pad.
 17. The method according to claim 12, wherein the fiber iscushioned along substantially the entire length of the fiber between theaffixing locations by a low-modulus damping material.
 18. The methodaccording to claim 12, wherein the substrate has a channel formed ateach of the affixing locations of the upper surface sized to receive thefiber at a temperature lower than any anticipated use of the device andto clamp on the fiber at a normal use temperature range, furthercomprising; cooling the substrate to the lower temperature; insertingthe fiber in each channel; and warming the substrate to the normal usetemperature range to clamp the fiber.
 19. The method according to claim10, in which the beta-eucryptite comprises SiO₂ 43-55%, Al₂O₃ 31-42%,Li₂O 8-11%, TiO₂ 2-6%, and ZrO₄ 0-4% on a weight percent basis.
 20. Amethod for producing an athermal optical fiber grating devicecomprising: providing a negative expansion substrate having an uppersurface and first and second ends; mounting a photorefractive-sensitiveoptical fiber onto the substrate upper surface; applying a sufficienttension to the optical fiber to maintain the fiber under tension at allanticipated use temperatures; affixing the optical fiber to thesubstrate at at least two spaced apart locations; and exposing theoptical fiber to UV light in order to define at least one opticalgrating therein such that the grating lies between and at a distancefrom each end either before or after the mounting step.
 21. The methodaccording to claim 20, wherein the substrate comprise a beta-eucryptiteglass-ceramic.
 22. An athermal optical device comprising: a negativeexpansion substrate having an upper surface, said substrate comprising abeta-eucryptite glass-ceramic; a thermally sensitive, positive expansionoptical component affixed to the substrate upper surface at at least twospaced apart locations.
 23. The device according to claim 22, in whichthe beta-eucryptite comprises SiO₂ 43-55%, Al₂O₃ 31-42%, Li ₂O 8-11%,TiO₂ 2-6%, and ZrO₄ 0-4% on a weight percent basis.
 24. An athermaloptical device comprising: a negative expansion substrate having anupper surface; and a thermally sensitive, positive expansion opticalcomponent affixed to the substrate upper surface at at least two spacedapart locations; wherein the optical component is an optical fibercoupler, the coupler comprising at least two optical fibers fusedtogether at one or more points along their lengths.
 25. An athermaloptical fiber grating device comprising; a negative expansion substratehaving an upper surface and first and second ends, said substratecomprising a beta-eucryptite glass-ceramic; an optical fiber affixed tothe substrate upper surface at at least two spaced apart locations; anda grating defined in the optical fiber between and at a distance fromeach substrate end.
 26. The device according to claim 25, in which thebeta-eucryptite comprises SiO₂ 43-55%, Al₂O₃ 31-42%, Li₂O 8-11%, TiO₂2-6%, and ZrO₄ 0-4% on a weight percent basis.
 27. The device accordingto claim 26, in which the beta-eucryptite has a negative coefficient ofthermal expansion between −30×10⁻⁷/° C. and −90×10⁻⁷/° C.
 28. Anathermal optical fiber grating device comprising; a negative expansionsubstrate having an upper surface and first and second ends; an opticalfiber; and a grating defined in the optical fiber between and at adistance from each substrate end; wherein: (i) the optical fiber isaffixed to the substrate upper surface at first and second spaced apartlocations, the first location is between the grating and the firstsubstrate end and the second location is between the grating and thesecond substrate end; and (ii) the device further comprises a lowmodulus damping material connected to substantially the entire length ofthe fiber between the affixing locations.
 29. An athermal optical fibergrating device comprising; a negative expansion substrate having anupper surface and first and second ends; an optical fiber; and a gratingdefined in the optical fiber between and at a distance from eachsubstrate end; wherein: (i) the optical fiber is affixed to thesubstrate upper surface at first and second spaced apart locations, thefirst location is between the grating and the first substrate end andthe second location is between the grating and the second substrate end;and (ii) each affixing location comprises a channel sized to receive thefiber at a temperature lower than any anticipated use of the device andto clamp on the fiber at normal use temperatures.
 30. A method forproducing an athermal optical fiber grating device comprising: (a)providing a negative expansion substrate having an upper surface andfirst and second ends; (b) mounting an optical fiber with at least onegrating defined therein onto the substrate upper surface such that thegrating lies between and at a distance from each end; and (c) affixingthe optical fiber to the substrate at at least two spaced apartlocations; wherein: (i) in step (a), the negative expansion substrate isselected to provide thermal compensation to the grating; and (ii) themethod further comprises applying a sufficient tension to the opticalfiber before the affixing step to maintain the fiber under tension atall anticipated use temperatures.
 31. A method for producing an athermaloptical fiber grating device comprising: (a) providing a negativeexpansion substrate having an upper surface and first and second ends;(b) mounting an optical fiber with at least one grating defined thereinonto the substrate upper surface such that the grating lies between andat a distance from each end; and (c) affixing the optical fiber to thesubstrate at at least two spaced apart locations; wherein: (i) in step(a), the negative expansion substrate is selected to provide thermalcompensation to the grating; (ii) the optical fiber is affixed to thesubstrate upper surface at a location between the grating and the firstend and at a location between the grating and the second end; and (iii)the fiber is cushioned along substantially the entire length of thefiber between the affixing locations by a low-modulus damping material.32. A method for producing an athermal optical fiber grating devicecomprising: (a) providing a negative expansion substrate having an uppersurface and first and second ends; (b) mounting an optical fiber with atleast one grating defined therein onto the substrate upper surface suchthat the grating lies between and at a distance from each end; and (c)affixing the optical fiber to the substrate at at least two spaced apartlocations; wherein: (i) in step (a), the negative expansion substrate isselected to provide thermal compensation to the grating; (ii) theoptical fiber is affixed to the substrate upper surface at a locationbetween the grating and the first end and at a location between thegrating and the second end; (iii) the substrate has a channel formed ateach of the affixing locations of the upper surface sized to receive thefiber at a temperature lower than any anticipated use of the device andto clamp on the fiber at a normal use temperature range; and (iv) themethod further comprises: (1) cooling the substrate to the lowertemperature; (2) inserting the fiber in each channel; and (3) warmingthe substrate to the normal use temperature range to clamp the fiber.